Electrolytic protective coating for magnesium



United States Patent ELECTROLYTIC PROTECTIVE COATING FOR MAGNESIUM William McNeill, Philadelphia, Pa., assignor to the United States of America as represented by the Secretary of the Army No Drawing. Application June 2, 1954, Serial No. 434,093

7 Claims. c1. 20456) (Granted under Title 35, U. S. Code (195E), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to a method of and baths for protecting magnesium and magnesium base alloys against corrosion, and has among its numerous objects the provision of chromate-based aqueous baths in which magnesium or its alloys may be electrolytically treated to form a corrosion-inhibiting coating thereon.

The prior art 'has witnessed attempts to protect magnesiurn and its alloys anodically or electrolytically by means of both acid and alkaline baths containing chromates together with phosphates, fluorides and other salts of alkali metals. I now have found that far superior protective coatings can be obtained by employing aqueous baths containing chromates, phosphates, fluorides, and either ammonia or an organic base such as choline, hexamethylenetetramine, or pyridine. In these superior baths I have found it to be exceedingly beneficial to the coatings obtainable therewith to exclude all alkali metal ions therefrom.

In accordance with the present invention, the following is a specific example of the methods by which my novel and superior protective coating may be produced on magnesium and magnesium base alloys. The object to be protected is first suitably cleaned, and then is treated in an electrolytic bath solution containing from 0.25 to 2.25 M (moles/liter) of phosphoric acid, from 0.30 to 4.00 M of hydrofluoric acid, from 0.05 to 0.7 M CrOs, and a sufii cient amount of an alkalinity agent such as NHsOH or equivalent organic base to adjust the pH of the solution to within a range of from about 5.5 to about 9.

The magnesium articles to be coated are made the electrodes and alternating current is applied therethrough to the bath. Direct current may also be employed, but is not preferred because somewhat greater difficulty is encountered in obtaining coatings therewith. Preferably, articles of approximately equal surface area should be connected to each bath terminal in the A. C. operation. Suitable operating conditions for the electrolytic treatment are a current density of from l2-15 amps/ft. (based on total work area both electrodes), a temperature of at least 25 C., and a treatment time of from 10 to 20 min utes. As a rule it is preferable to keep the product of the time and current density constant, so in case one is raised or lowered it is best to inversely lower or raise the other. Actually, however, the current density can be varied widely; e. g., I have used as low as 5 amps/ft. and as high as 200 amps/ft. with success, and undoubtedly higher current densities could be used so as to speed up the process. The completion of the treatment, as measured in terms of the attainment of a coating having the desired corrosion resistance, can be ascertained by noting the colored coatings may result.

2,778,789 Patented Jan. 22, 1957 in accordance with ASTM Test 'B-ll744T (American Society for Testing Materials), only a relatively few corrosion spots were visible to the unaided eye. A relatively high dielectric strength, averaging in most cases between 300-500 volts, was detected when measured in accordance with ASTM Test B-'-l10-45. When the baths including ammonia are employed, green coatings are produced ranging from light to dark shades approaching black, as the amount of the chrom'ate is increased. If, instead of ammonia, one of the organic bases is employed, different For example, baths with choline (trimethylhydroxyethyl ammonium hydroxide) and hexamethylenetetramine produced a green coating similar to the ammoniacal baths, but triethanolamine baths will cause a black or 'dark grey coating, and ethylenediamine baths produce black coatings.

In seeking to determine the possible interchangeability of other bases for the ammonia and organic bases which gave the excellent results in the baths and processes described above, sodium, potassium and cesium hydroxides were tried. 'lihe'results were so significantly inferior as to indicate that the substantially complete absence of alkali metal ions from the baths of the present invention is important to their success. A clue to the remarkably different effect which the presence of these ions causes is noticeable in the course of the electrolysis, for the terminating voltages are far lower than that with any containing either ammonia or an organic base, and increasing the time of the operation does not result in appreciably higher voltages. The results of using baths with these ions are even more spectacular by way of contrast with the coatings made by my novel baths which are essentially devoid of alkali metal ions. Instead of the smooth, thin, compact and hard coatings which feature my inventive protective treatment, rough, thick and very porous coatings which, when freshly prepared, can be scratched easily with a fingernail, and which have a markedly decreased corrosion resistance, are obtained when sodium, potassium or cesium ions are introduced into the bath.

This deleterious effect of the alkali metal ions has imposed a requirement, for the best possible practice of my present invention, to omit them not only from the base used to neutralize the acid components of the bath but also from the other constituents which make up the bath. Thus, the phosphate and fluoride ions employed, for example, should either be in the form of phosphoric or hydrofluoric acids, or ammonium phosphate, ammonium fluoride, etc., but never sodium or potassium phosphate or fluoride.

The methods and baths described herein are subject to considerable variation within the skill of the art, and I accordingly do not wish to be limited to my illustrative descriptions thereof beyond what is encompassed by the following claims.

I claim:

1. A method of producing hard and compact protective coatings on magnesium and magnesium-base alloys which comprises making the magnesium articles the electrodes and electrolytically treating them under progressively increasing potential to a selected terminating voltage of any amount up to just over 300 volts, depending on the quality of the coating desired, in an aqueous solution whose solute ingredients are a chromate, a phosphate and a fluoride that yield non-metallic cations only, and

whose pH is maintained in a range of about 5.5 to 9.0 with an" alkalinity agent yielding non-metallic cations only.

2. A method of producing hard and compact protective coatings on magnesium and magnesium-base alloys which comprises making the magnesium articles the electrodes and electrolytically treating them under progressively increasing potential to a selected terminating voltage of any amount up to just over 300 volts, depending on the quality of the coating desired, in anaqueous solution whose solute ingredients are chromic, hydrofluoric and phosphoric acids, and whose pH is maintained in a range of about 5 .5 to 9.0 with an alkalinity agent yielding solely non-metallic cations.

3. A method of producing hard and compact protective coatings on magnesium and magnesium-base alloys which comprises making the magnesium articles the electrodes and electrolytically treating them under progressively increasing potential to a selected terminating voltage of any amount up to just over 300 volts, depending on the quality of the coating desired, in an aqueous solution Whose solute ingredients are chromic, hydrofluoric and phosphoric acids, and whose pH is maintained in a range of about 5.5 to 9.0 with an agent selected from a group consisting of ammonium hydroxide, choline, pyridine, hexamethylenetetramine, triethanolamine and ethylenediamine.

4. A method of producing hard and compact protective coatings on magnesium and magnesium-base alloys which comprises making the magnesium articles the electrodes and electrolytically treating them under progressively increasing potential to a selected terminating voltage of any amount up to just over 300 volts, depending on the quality of the coating desired, in an aqueous solution whose solute ingredients are chromic, hydrofluoric and phosphoric acids, and whose pH is maintained in a range of about 5.5 to 9.0 with ammonia.

5. A method of producing hard and compact protective coatings on magnesium and magnesium-base alloys which comprises making the magnesium articles the electrodes and electrolytically treating them under progressively increasing potential to a selected terminating voltage of any amount up to just over 300 volts, depending on the quality of the coating desired, in an aqueous solution whose solute ingredients are chromic acid in a concentration of 0.05 to 0.70 mole per liter, hydrofluoric acid in a concentration of 0.30 to 4.00 moles per liter, and phosphoric acid in a concentration of .25 to 2.25 moles per liter, and whose pH is maintained in a range of about 5.5 to9.0 with ammonia.

6. A method of producing hard and compact protective coatings on magnesium and magnesium-base alloys which comprises making the magnesium articles the electrodes and electrolytically treating them under progressively increasing potential to a selected terminating voltage of any amount up to just over 300 volts, depending on the quality of the coating desired, in an aqueous solution Whose solute ingredients are chromic acid in a concentration of 0.15 to 0.40 mole per liter, hydrofluoric acid in a concentration of 0.3 to 0.8 mole per liter and phosphoric acid in a concentration of 0.4 to 0.8 mole per liter, and Whose pH is maintained in a range of about 5.5 to 9.0 with ammonia.

7. A method of producing hard and compact protective coatings on magnesium and magnesium-base alloys which comprises making the magnesium articles the electrodes and electrolytically treating them at a current density of about 12 to 15 amperes per sq./ft. based on total area of the electrodes, under progressively increasing potential to a selected terminating voltage of any amount up to just over 300 volts, depending on the quality of the coating desired, at a temperature of at least 25 C. in an aqueous solution whose solute ingredients are chromic acid in a concentration of 0.15 to 0.40 mole per liter, hydrofluoric acid in a concentration of 0.3 to 0.8 mole per liter, and phosphoric acid in a concentration of 0.4 to 0.8 mole per liter, and whose pH is maintained in a range of about 5.5 to 9.0 with ammonia.

References Cited in the file of this patent FOREIGN PATENTS 543,726 Great Britain Mar. 10, 1942 502,678 Great Britain Mar. 22, 1939 815,155 France Apr. 5, 1937 

1. A METHOD OF PRODUCING HARD AND COMPACT PROTECTIVE COATINGS ON MAGNESIUM AND MAGNESIUM-BASE ALLOYS WHICH COMPRISES MAKING THE MAGNESIUM ARTICLES THE ELECTRODES AND ELECTROLYTICALLY TREATING THEM UNDER PROGRESSIVELY INCREASING POTENTIAL TO A SELECTED TERMINATING VOLTAGE OF ANY AMOUNT UP TO JUST OVER 300 VOLT, DEPENDING ON THE QUALITY OF THE COATING DESIRED, IN AN AQUEOUS SOLUTION WHOSE SOLUTE INGREDIENTS ARE A CHROMATE, A PHOSPHATE AND A FLUORIDE THAT YIELD NON-METALLIC CATIONS ONLY, AND WHOSE PH IS MAINTAINED IN A RANGE OF ABOUT 5.5 TO 9.0 WITH AN ALKALINITY AGENT YIELDING NON-METALLIC CATIONS ONLY. 